Step-scanning system for mass spectrometer

ABSTRACT

A single-focusing, 60* sector magnet mass spectrometer constructed with symmetric conjugate foci calculated from fringe field data and corresponding to a beam deflection of 68* is provided with a programmable magnetic field to step scan spectral lines and &#39;&#39;&#39;&#39;zero&#39;&#39;&#39;&#39; lines on both sides of each spectral line. A rotating coil probe in the magnetic field, and a stationary coil around a magnetic pole face used to provide the field, are employed as field magnitude and time-rate-of-change sensors for a current nulling system. The nulling system cooperates with an automatic step-scan programmer to set the magnetic field to 3N values, where N is the number of spectral lines to be step scanned, and the successive values correspond to the centers of spectral lines and &#39;&#39;&#39;&#39;zeros&#39;&#39;&#39;&#39; on both sides of each spectral line. The programmer selects a reference voltage for each spectral line which is compared with an induced voltage in the rotating coil. The difference drives the electromagnet power supply to rapidly change the field. The change induces a rate signal in the stationary coil to produce a damping signal for the nulling system. At each step, a data acquisition system records in digital form a value proportional to ion current, scale factor, and the integrating time for that value to be developed through an integrating digital voltmeter. A computer then receives the data for later analysis.



2. Apparatus as defined in claim 1 wherein: said voltage producing means comprises means for producing N unique voltages selected to provide through control of said sweep system magnetic fields which will focus ions having the same mass values at said common point, where said N values correspond to said values at different ones of said centers of N mass peaks, and means for providing incremental voltages to be added and subtracted from each of said unique voltages to provide for each of said N unique voltages produced a triad of reference voltages; and said sequencing means comprises means for selecting in sequence said N unique voltages, one unique voltage for every three successive cycles of said measuring means, and means for sequentially subtracting, neither subtracting nor adding, and adding different ones of said incremental voltages to each of said N unique voltages in a given order to provide said triads of reference voltages.
 3. Apparatus as defined in claim 2 wherein said stabilizing means comprises: means for monitoring the flux of said field and producing a first voltage signal having a value proportional to the magnitude of said flux; means for subtracting said first voltage signal from said reference voltage to produce a control signal for control of said voltage controlled magnetic field sweep system; means for monitoring the time rate of change of said flux to produce a second voltage signal having a value proportional to said rate of change and a polarity corresponding to the sign of said change; and means responsive to said second voltage signal for damping oscillations in said control signal.
 4. Apparatus as defined in claim 3 wherein said measuring means provides an output signal proportional to ion beam current in digital form by integration for a predetermined period of time, and including means for storing said output signal and for simultaneously storing a signal in digital form representing the time of integration.
 5. Apparatus of claim 4 wherein said measuring means includes means for scaling said output signal, and said storing means simultaneously stores with each output signal a corresponding scaling factor.
 6. Apparatus of claim 5 including a computer connected to said storing means to receive stored data for online processing.
 7. In a system for mass spectroscopy having an electromagnet and a collector slit, apparatus comprising; means for selectively focusing said ion beams to pass through said slit by successively altering the magnetic field of said electromagnet from one value to another at a high rate in a stepwise fashion for each peak of a mass spectrum and for valleys on boTh sides of each peak to be measured; means for determining when a static magnetic field has been reached by said focusing means; and means responsive to said determining means for initiating measurement of ion intensity when a static magnetic field has been reached.
 8. Apparatus as defined by claim 7 wherein said magnetic field values are selected from predetermined values in successive order from one extreme value to another.
 9. Apparatus as defined by claim 8 including: programming means for selectively predetermining magnetic field values to be selected; and means for timing a period for measurement of ion intensity at each magnetic field value.
 10. In a system for mass spectroscopy having an electromagnet including a gap, and an exciting coil producing a magnetic field in said gap for focusing selected ion beams at the centers of peaks of a spectrum through a collector slit, the combination comprising: means for producing predetermined reference signals in a stepwise fashion, one reference signal for each selected ion beam; means responsive to said reference signals for producing field control signals used for controlling current through said exciting coil to establish for each reference signal a triad of magnetic fields of predetermined values in said gap corresponding to focus ions at centers of said selected ion beams to pass through said slit and focus ions on both sides of said selected ion beams to pass through said slit in order to provide background reference measurements on both sides of each selected ion beam against which the true amplitude of a selected ion beam the reference measurements may be determined; field sensing means for producing a feedback signal proportional to said magnetic field; means responsive to a given field control signal and said feedback signal for producing an error signal equal to the algebraic difference therebetween; and a controlled power supply means connected to said exciting coil for varying current through said coil in response to the amplitude and polarity of said error signal in a direction to reduce said error signal.
 11. The combination as defined in claim 10 including a rate coil wound around a pole of said electromagnet, the axis of said coil being substantially parallel to flux of said field at the center of said pole, whereby a signal is produced in said rate coil of a polarity and amplitude corresponding to the time rate of change and direction of change of said field, and mans combining said rate signal with said error signal for damping oscillations of said error signal.
 12. In a system for mass spectroscopy having an electromagnet for focusing selected ion beams through a collector slit, said electromagnet having an exciting coil, the combination comprising: means for producing an analog signal as a function of said ion beam intensity focused onto said collector slit; means for selecting a period of integration of said ion beams; means for integrating said analog signal for said selected period and for developing in digital form an output signal having a value proportional to the integral of said analog signal over said period; a buffer memory; means for transferring to said buffer memory said output in digital form together with the value of the period selected in digital form after said period has lapsed; and programming means for altering the level of magnetic field current through said exciting coil in a stepwise fashion to rapidly focus another ion beam through said collector slit.
 13. The combination as defined in claim 12 wherein said programming means is preset to focus the centers of ion beams of selected mass numbers through said collector slit for spectral line intensity analysis and at separate times both sides of each ion beam of selected mass numbers, thereby providing background level analysis to be used in spectral line intensity analysis.
 14. The combination as defined in claim 12 wherein said progrAmming means is preset to focus the centers of ion beams of selected mass numbers through said collector slit by programmed reference signals, and preset to focus the ion beams on both sides of the centers of each ion beam corresponding to a selected mass number by at one time subtracting and another time adding predetermined signal increments to each of said reference signals.
 15. The combination of claim 14 wherein said programmed reference signals are selected by a first switching means, and predetermined signal increments are added and subtracted by a second switching means, said second switching means being actuated each time said third means transfers a digital output signal to said buffer memory to subtract an incremental signal at one time, to add an incremental signal at another time, and neither add nor subtract an incremental signal at a time intermediate said one time and said another time for a given reference signal corresponding to one mass number. 